What is 50m, 60m, 70m, and 80m javelins?

I have been looking into buying a javelin so that i can continue to practice in the off season from track. I am in high school and i know that the standard weight is 800 grams but on all sites selling javelins it allso lists whether it is 50m, 60m, 70m, or 80m. I have no idea what this means so im wondering if anyone knows what the highschool regulation is for whatever this is.